feat(agent): run the service at MCS level s0:c1023 on SELinux systems

doc/policy-types/selinux.md

@@ -145,7 +145,12 @@ inapplicable item that names it.
     parentheses and a statement allowlist; `semodule` on the node reports
     anything else. A Bor scheduled task can run under a type declared in a
     custom module: set `SELinuxContext` on the task in a
-    [Services & timers](systemd.md) policy.
+    [Services & timers](systemd.md) policy. The agent process itself runs
+    in the unconfined service domain at MCS level `s0:c1023` (see
+    [Hardening](../security/hardening.md#built-in-service-hardening)), so
+    a custom module can single it out by level, for example with a
+    `constrain` statement that refuses signals from senders whose level
+    does not dominate the target's.
 
 **File contexts**
 :   A path expression (`/opt/corpapp(/.*)?`), the SELinux type, the file

doc/security/hardening.md

@@ -143,6 +143,17 @@ The agent (`bor-agent.service`) runs as root. It must write system policy
 files (browser policies, dconf, polkit rules, firewalld zones, repository
 definitions and so on), so it is not sandboxed.
 
+On SELinux systems the unit starts the agent in the unconfined service
+domain at MCS level `s0:c1023` (`SELinuxContext=` in the unit; the leading
+`-` lets the agent start where the policy has no such type, and the
+directive is ignored where SELinux is disabled). The category confines
+nothing by itself. It lets an [SELinux](../policy-types/selinux.md) policy
+single the agent process out, for example with a constraint that refuses
+signals from senders at a lower level, so that `kill -9` from a root shell
+is denied while `systemctl restart bor-agent` keeps working. Files the
+agent writes carry the level; on the targeted policy MCS binds only
+container domains for file access, so nothing else is affected.
+
 ## File permissions
 
 | Path | Owner | Mode | Notes |

packaging/systemd/bor-agent.service

@@ -13,6 +13,15 @@ RestartSec=30
 # cancelled through the agent's context on stop; give them time to wind down
 # cleanly before systemd escalates to SIGKILL.
 TimeoutStopSec=120
+# Run the agent with an MCS category of its own on SELinux systems. The
+# category changes nothing by itself: the agent stays in the unconfined
+# service domain and MCS binds only container domains for file access. It
+# lets an SELinux policy single the agent process out, for example a
+# constraint that refuses signals from senders at a lower level, without a
+# policy module per node. The leading "-" makes systemd ignore a failure to
+# set the context (a policy without the type); the directive is ignored
+# entirely where SELinux is disabled.
+SELinuxContext=-system_u:system_r:unconfined_service_t:s0:c1023
 StandardOutput=journal
 StandardError=journal
 SyslogIdentifier=bor-agent